MARTIN
REES: In the last few years the problem of understanding the ultra-early
universe has come into focus. We now know the key properties of
the universe—its density, its age, and its main constituents.
Indeed the last three years will go down as specially remarkable
in the annals of cosmology because just within these years we've
pinned down the shape and contents of the universe, just as in earlier
centuries the pioneer navigators determined the size of the earth
and the layout of its continents. The challenge now is to explain
how it got that way. The new physics is attempting to understand
why it's expanding the way it is, and why it ended up with the content
it has. We can trace its history back to about a micro-second after
the putative 'big bang' that started it off, but what happened in
that first, formative microsecond? The boisterous variety of ideas
being discussed—branes, inflation, etc.—makes clear that
the issues are fascinating, but also we're still a long way from
the right answer. We're at the stage where all possibilities should
be explored. It's worthwhile to consider the consequences of even
the most flaky ideas, although the chance of any of them actually
panning out in the long run is not very high.

In
my own work, I try to be open to several ideas at once (even if
they're incompatible) because I want to know the answer. If a phenomenon
is puzzling, it's a good idea to explore all options: you'll thereby
perhaps find new ways to discriminate among them, or else further
study may reveal contradictions that rule some of them out. Obviously,
the community collectively does that, but individual scientists
fall into two categories. Some individuals aren't motivated to work
on a theory unless (at least at the time) they feel pretty convinced
it's likely to be correct—they put all their money on a particular
horse. But other scientists (and I'm in this second category) are
happy to spread their bets, and find the wish to clarify the issue
itself a sufficient motivation.

I
wouldn't claim to be a technical expert in any of the specific theories
for the ultra early universe. It seems likely that extra dimensions
of space are going to play a role; it's very good that the idea
of inflation, which has dominated the field for 20 years, is now
being generalized by other concepts that have come from people like
Lisa Randall, Neil Turok, and Paul Steinhardt. It's important to
explore all of these avenues.

The
key goal, of course, is to develop a convincing, all-encompassing
theory that describes the early universe and that makes testable
predictions about the world today. If we had a theory that gave
us a deeper and more specific understanding of the masses of electrons
and protons, and of the forces governing them than the so-called
'standard model' does today, then that theory would gain credibility,
and we'd take seriously its implications for the ultra-early universe
. The hope is that one of the exotic new theories will make testable
predictions either about the ordinary world of particles, or about
the universe. For instance, some make distinctive predictions about
the amount of gravitational radiation filling the universe. We can't
yet measure this today but within ten years we might be able to
do it. That's one way in which astronomical observations might be
able to narrow down the range of options.

The
easiest idea to understand conceptually is eternal inflation, which
Guth advocates and on which Andrei Linde has done a great deal of
detail work. This naturally gives rise to many Big Bangs. Whether
or not those Big Bangs will be close replicas of each other, or
whether the material in each of them would be governed by different
laws is something we don't know. Eternal inflation may bypass the
complications of extra dimensions and quantum gravity, because these
are relegated to the infinite past.